1. Field
This disclosure relates generally to semiconductors, and more specifically, to semiconductor read-only-memories (ROMs) and the programming thereof.
2. Related Art
An integration technology known as 3D (three-dimensional) integration technology uses the stacking of layers of various types of circuits in a single integrated circuit in order to reduce the overall circuit footprint. One application of 3D integration technology is for semiconductor memories or memory plus logic circuitry. Such 3D technologies however involve complex semiconductor manufacturing techniques.
Read-only-memories (ROMs) that are programmable are commonly implemented in integrated circuits. Such memories are programmed either after manufacturing, known as field programmable, or during the manufacture of the memory and known as mask ROMs. One type of memory that is programmed after manufacturing is a field programmable memory and typically uses fuses or antifuses. Field programmable memories require additional circuitry that is not needed in mask ROMs and the extra circuitry consume circuit area and increase cost. For example, implementing the field programming involves high programming voltages. Additionally the field programming is slow and consumes significant test equipment time. Memories that are programmed during manufacture by use of a programming mask avoid the expense of the slow programming of field programmable ROMs. A disadvantage however is that the integrated circuit is analogous to a custom integrated circuit and requires unique processing and handling. Inventory control must therefore be more carefully monitored for this type of product. Additionally, the amount of manufacturing time between the mask programming step of ROMs and the completion of the integrated circuit may be an issue to customers if the programming occurs relatively early in the manufacturing process.
In order to increase the packaging density associated with semiconductor memories, others have implemented multiple chip memory modules. Such modules are typically formed of dynamic random access memories (DRAMs) which are not programmable. If programmable memories are implemented, such memories are typically FLASH memories. However, FLASH memory modules have a significant cost as compared with ROM and have less data security than ROM products.